Combustion product condensing water heater

ABSTRACT

In a water heating system, vapor in the products of combustion gases is condensed in a secondary heat exchanger positioned in a housing with the primary heat exchanger and combustion chamber. The two heat exchangers are coaxial coils with the secondary coil positioned below the primary. Gases flow radially through the primary coil and then axially through the secondary coil at an increased velocity. The gases are then used to pre-heat a gas/air mixture in a third heat exchanger within the secondary heat exchanger. The pre-heated gas/air mixture is burned in a burner within the primary heat exchanger and the gas products are drawn through the exchangers by a blower. A water storage tank is designed to enhance stratification of hot water over cooler water. The cooler water is used to condense vapor in the secondary heat exchanger.

FIELD OF THE INVENTION

This invention relates to water heaters. More particularly, it relatesto water heaters in which vapor in the products of combustion iscondensed to retrieve latent heat of vaporization.

BACKGROUND

With the increasing cost of fuel, methods for increasing the efficiencyof heat exchanger assemblies for extracting heat from the products ofcombustion of fuel burners has become increasingly more cost effective.One means of increasing the efficiency of heat recovery has been to burnthe fuel in small-volume, water-walled combustion chambers. Forced draftor pulsed combustion techniques are utilized to achieve high rates ofheat transfer and to vent the products of combustion.

Recently, systems have been proposed to cool the products of combustionbelow the dew point temperature of those products, typically below 130°F., in order to condense some of the vapor and thereby extract thelatent heat of vaporization of that vapor. To cool the products ofcombustion to that extent and to minimize the size, and thus the cost,of the heat exchanger assemblies, efficient heat exchangers must bedesigned.

An object of this invention is to provide an efficient water heatingsystem in which heat is extracted from the products of combustion bycondensing vapor in the products, the system having an acceptable sizeand cost.

The condensate from natural gas combustion products is mildly acidic,and the acidic nature of the condensate is expected to be a potentialcause of corrosion. The acidic nature of the condensate may result fromsulfuric acid, nitric acid, and carbonic acid.

A further object of this invention is to provide a water heating systemdesigned to withstand the corrosive effects of the condensate at aminimal cost.

DISCLOSURE OF THE INVENTION

In a heating assembly of this invention, primary and secondary heatexchangers and a combustion chamber are positioned within a singlehousing. The combustion chamber is defined by the primary heatexchanger. The combustion products flow through the primary heatexchanger at a sufficiently low velocity to keep the temperature of theheat exchanger walls at an acceptable level. The products of combustionare then directed into a secondary heat exchanger in which the velocityof the products of combustion is increased in order to increase the heattransfer coefficient of that heat exchanger. Cold water flows throughthe secondary heat exchanger in counter flow relationship with thecombustion gases to cool those products to a temperature below the dewpoint temperature. Vapor in the products of combustion is therebycondensed. After being heated in the secondary heat exchanger, the wateris mixed with hot water from the output of the primary heat exchangerand the water mixture is directed through the primary heat exchanger ata higher flow rate than in the secondary heat exchanger. The hot mixtureof water entering the primary heat exchanger assures that nocondensation of the products of combustion occurs at this heatexchanger.

In the preferred water heating assembly, the primary heat exchanger is acoil of tubing which defines the combustion chamber. Products ofcombustion flow radially through that coil. The secondary heat exchangeris a second coil of tubing coaxial with but lower than the first.Products of combustion flow through that coil axially at a highervelocity. Prior to combustion, the combustion air and fuel may bepreheated by combustion products in a tertiary heat exchanger whichreceives those products from the secondary heat exchanger.

In a system in which hot water is stored in an insulated storage tank,cool water is taken from the bottom of the tank and introduced into theburner and heat exchanger assembly, and the heated water is returned toan upper section of the storage vessel in such a manner as to avoidmixing of the heated water with the cooler water in the bottom of thevessel.

The preferred system utilizes a blower downstream of the burner and heatexchanger assembly for inducing a draft to propel the products ofcombustion.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a block diagram of the water circuit of a preferred systemembodying this invention;

FIG. 2 is an elevational cross sectional view of a preferred embodimentof the burner and heat exchanger assembly embodying this invention;

FIG. 3 is an elevational cross sectional view of a possible storage tankconfiguration for use with this invention;

FIG. 4 is an elevational cross sectional view of an insulated plasticlined storage tank for use in this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred system embodying this invention is shown schematically inFIG. 1. A storage tank 12 is connected to an external gas-fired waterheater 14 by supply and return pipes 16 and 18. The water heater 14comprises a primary, fired heat exchanger assembly 20 and a economizer22 which is a secondary heat exchanger operating in the condensing mode.A circulating pump 24 placed between the economizer and the primary heatexchanger draws water from the return line 18 and from the economizer 22and drives that mixture through the primary heat exhanger 20.

The storage tank is designed to maximize stratification between a smallvolume 32 of relatively cool water in the bottom of the tank and alarger volume 34 of stored hot water. In this case, the two volumes areseparated by baffles 33. Typically the cool volume 32 is about 20percent of the total tank volume. If hot water is taken from the outlet38 of the storage tank while the heater 14 is in the standby mode, coldwater is introduced into the lower volume 32 through a diffuser from acold water inlet 31 and pipe 35.

When the heater assembly 14 is turned on, water is drawn from the lowervolume 32 through pipe 16 if no water is being extracted through outlet38; or a mix of water from the cold water inlet 31 and the lower volume32 is drawn through pipe 16 if water is being extracted from the outlet38. Water heated in the heater assembly 14 is returned through pipe 18to the upper volume 34 in the storage tank 12. The baffles 33 inhibitmixing of the hot water from pipe 18 with the cooler water in the volume32.

The cool water introduced into the economizer 22 through pipe 16 passesin counter flow heat exchange relationship with products of combustionwhich have already been cooled somewhat in the primary heat exchanger20. The products of combustion and the water are sufficiently cool whenintroduced into the economizer 22 that the temperature of the productsof combustion within the economizer is below the dew point temperature.This results in condensation of vapor in the products of combustion, andthe latent heat of vaporization is transferred to the water in theeconomizer.

Water which has been preheated in the economizer 22 is introduced intothe primary heat exchanger 20 which defines a combustion chamber. Therethe water is heated to the high temperature necessary for heating thewater in the storage tank 12.

The purpose of the cold volume 32 should now be apparent. It provides asufficiently large reservoir of cool water to enable the economizer tooperate in the condensing mode throughout the on-cycle even when no coldwater is drawn through the inlet 31 during the heating cycle. The volumeof cool water should be minimized to reduce standby losses from and tolimit the size of the storage tank. To that end, the cool water isrationed to the heater 14 at a low flow rate to condense vapor in theexhaust gas with a minimal amount of water.

The percentage of the storage tank which must be devoted to the volumeof cooler water 32 can be determined from the following equation:##EQU1## Where V_(H) and V_(C) are the respective hot and cool volumes34 and 32, T_(R) is the temperature of the water in return line 18,T_(cut-in) is the temperature of water in the storage tank at which thewater heater is fired, and T_(Diff) is the differential between thethermostat cut-in and cut-off temperatures. Typically, T_(R) -T_(cut-in)is in the range of 40° to 50° F. To minimize standby losses and totaltank volume, the volume V_(C) should be less than 20 percent of thetotal tank volume. Thus, the thermostat differential temperature must beless than about 10° F. A temperature differential of 5° to 10° F. and acool volume of 10 to 20 percent of the total tank volume are reasonable.For a given burner input, the flow rate through the heater can becontrolled by a thermostatic valve 37 to maintain the desired returntemperature. Alternatively, the flow rate might be held constant and theburner input vaired, to maintain the steady return temperature.

Placement of pump 24 between primary heat exchanger 20 and the secondaryheat exchanger 22 is important for the following reason. Hot water fromthe outlet 28 of the primary heat exchanger is recirculated back to theinlet 30 to raise the water inlet temperature of that heat exchangerabove the dew point temperature of the products of combustion. This isdone to prevent condensation in the primary heat exchanger. To minimizethe cost of the system, the primary heat exchanger is not protectedagainst corrosion by flue gas condensate.

A further advantage of recirculating water through the primary exchangeris that it increases water flow rate and thus establishes highwater-side heat transfer coefficients. This minimizes liming of the mainheat exchanger coil. This is unnecessary in the economizer due to thesignificantly lower heat fluxes and water temperatures in the economizersection.

The operating principle is best illustrated by the following example:Consider a 100 gallon tank with a thermostat that operates over a 10° F.differential and is located one fifth of the way from the bottom of thetank. Assume that the lower section 32 of the tank contains 20 gallonsof water at an average temperature of 80° F, and that the averagestorage temperature is 140° F.

In the proposed concept, the water heater would use the 20 gallons ofcooler water to heat 80 gallons of stored water from 135° F. to 145° F.During this process, the cooler water would be displaced by 135° F.water. A heat balance indicates that the total energy required is 15,700Btu. If the heat output of the water heater is 157,000 Btu/hr, then theburner-on time is 6 minutes. In this case, the circulating pump 24 woulddraw water at the rate of 3.33 GPM from the bottom section and wouldreturn it to the top section at a temperature of 175° F. At the end ofthe on-cycle, the mixed temperature of the upper section will havereached 145° F., and the bottom section will contain water at 135° F.The flow control is preferably accomplished by thermostatic control ofthe return temperature to the tank by a valve 37 at this will preventexcessive temperatures if the bottom temperature, and thus the heaterinlet temperature, gets too high. Alternatively, the desired flow ratemay be set by a constant flow regulator or by a fixed orifice.

A variation of the concept might include the use of a separate, smallerpreheat tank instead of the integral volume 32.

A preferred design of the water heater 14 is shown in FIG. 2. Theprimary heat exchanger consists of an integrally finned copper tube coil42 surrounding the combustion chamber 44. This arrangement provides anefficient "water-wall" combustion chamber which minimizes combustionchamber heat losses and requires a minimal amount of refractoryinsulation 46 and 48. Moreover, with radial flow through the coil, thelarge area of the coil facing the combustion chamber provides relativelylow gas velocities. Such low gas velocities are necessary to preventexcessive wall temperatures due to the high temperature of thecombustion products.

A mixture of natural gas and air is burned at a burner 50 within thecombustion chamber. In a pipe 56 combustion air from an exterior inlet52 is mixed with natural gas from a pipe 53 and nozzle 54. The desiredair flow rate is established by fixed orifice 55. The mixture is drawninto the combustion chamber by a blower 58 positioned in the flue gasoutlet. Alternatively, the mixture can be propelled by a blower placedupstream of the burner.

Combustion gases are collected in an exhaust annulus 60 before passingthrough the economizer coil. The gas temperature at the annulus is inthe range of 250° to 400° F.

The combustion products are cooled further in the economizer coil 62which is designed for condensing operation. Because of the corrosiveproperties of the condensate, the economizer is made of acorrosion-resistant material, such as 70/30 cupronickel. The economizeris designed for cross-counterflow of the combustion products. With axialflow of gasses through this coil, the combustion products flow at highvelocities in order to achieve high heat transfer coefficients. Here,high gas-side transfer coefficients can be utilized without fear ofexcessive wall temperatures because both the gas and water temperatureare much lower than in the main heat exchanger. Most of the systemproduct of combustion pressure drop will occur in this section.

FIG. 2 also illustrates a third heat exchanger section 64. This is acounterflow pre-heater which uses the latent and sensible heat of theexhaust products to preheat the incoming gas/air mixture. The preheater64 is a compact arrangement positioned concentrically within theeconomizer coil 62. Exhaust gas which has passed through the economizeris directed up through a first annulus 66 and then back down through asecond annulus 68. The annuli are separated by a cylinder 69. Gas in theannulus 68 is in counter flow heat exchange relationship with theincoming mixture of natural gas and air. Any liquid which condenses fromthe exhaust gases in this preheater is collected in a reservoir 70.Also, any condensed liquid from the economizer 62 flows through holes 72in the cylinder 69 into that same reservoir. The collected liquid istaken off through an anit-syphon tube 74 to the drain pipe 40. Theanti-syphon tube insures that exhaust gas can not leak into thesurrounding area but allows condensate to be drained from the heater.

With sufficiently low incoming water temperatures, the preheater 64 isprobably unnecessary, since it will add less than 1% to the recoveryefficiency. However, when incoming water temperatures are high, as mayoccur in a hot water booster, the air preheater may product worthwhilesavings. The heat that can be recovered in this type of preheater islimited by the heat capacity of the incoming mixture. Typically,preheating the inlet mixture by 100° F. would increase efficiency byapproximately 2.5%.

Losses in the system are minimized through the use of several designfeatures. The combustion chamber is small and does not have a largeinactive surface exposed to flame temperatures. The first stage heatexchanger is small with little water inventory. The bottom surface ofthe combustion chamber forms the top of the air preheater, so that heatlosses in this direction will reenter the exhaust stream and increasethe heat recovered in the preheater. Insulation is shown on this surfaceonly for the protection of the metal surface which forms the bottom ofthe combustion chamber. This insulation may be eliminated if the exhaustgas can be utilized to cool this surface. The main "radiation" heat lossoccurs through the top of the combustion chamber and along the outsideshroud which contains the exhaust gas. Both these surfaces are insulatedwith insulation 78 to minimize these losses. The exhaust gas isrelatively cool by the time it reaches the bottom of the unit and thissurface need not be insulated.

Other significant features include the use of an induced draft blowerwhich causes the unit to operate under negative draft conditions; thus,sealing the heater is not as critical as it would be if the unit werepressurized. Also since the exhaust gas is cool at this point, a plasticblower may be used to reduce costs and improved performance over a sheetmetal blower.

The unit is shown using sealed combustion; that is, combustion air isdrawn directly from the building exterior and exhaust gas is blowndirectly to the exterior. A natural convection stack is not feasiblebecause of the low exhaust temperature. An alternative is the use of aconventional air intake plus exhaust through plastic pipe installed as aroof vent or through an unused chimney.

Alternative storage tanks are illustrated in FIGS. 3 and 4. The tank ofFIG. 3 is conventional, with the exception of the provisions forstratification described above. This tank is a glass lined steel tank 80surrounded by insulation 82 and a metal housing 84. The hot return pipe18 eenters through the side of the tank and directs the hot waterupwardly into the upper volume 34. The baffle 33 is positioned below thehot return pipe 18 to assist in the natural stratification of the hotand cool water within the tank. Holes 86 in the baffle allow fordisplacement of water through the baffle as necessary. A diffuser 88 ispositioned over the inlet pipe 35 so that flow between the volumes 32and 34 is not induced by introduction of cold water into the tank. Athermostat 36 controls the operation of the heater.

An alternative version of the tank is illustrated in FIG. 4. Thispreferred tank structure includes a plastic lining 90 surrounded byinsulation 92, such as foam insulation, and an outer steel tank 94. Allconnections to the interior of the tank are through a bottom plate 96.This arrangement includes the baffle 33 and the diffuser 88 as in theembodiment of FIG. 3. The thermostat 36 is also connected through thebottom plate 96 to a remote sensing bulb 98.

It should be noted that the baffle 33 is not absolutely necessary. Itmay be sufficient to have the hot return pipe outlet positionedsufficiently high within the tank that a lower volume of cool waterremains. Also, flow directors may be mounted directly to the outlet ofpipe 18 to avoid the need for a baffle 33 fabricated within the storagetank.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

We claim:
 1. A method of heating water and storing the hot water in aninsulated storage vessel comprising:permitting natural stratification ofthe water within the vessel such that cooler water collects at thebottom of the vessel; withdrawing cooler water from the bottom of thevessel and passing that water in heat exchange relationship withcombustion gases from a burner assembly to cool those gases below theirdew point temperature so as to extract some of the latent heat ofvaporization from the gases and to heat the water; returning the thusheated water to the storage vessel at a location in the storage vesselabove the cooler water and in a manner as to avoid mixing with thecooler water, the volume of the cooler water below the location at whichhot water is returned to the vessel being sufficiently large to providefor condensation of the combustion gases through substantially theentire heating cycle of the burner.
 2. A water heating systemcomprising:a water-walled combustion chamber defined by a primary heatexchanger, the products of combustion gas flow path past the primaryheat exchanger providing for a first gas velocity through the primaryheat exchanger; a secondary heat exchanger for receiving gases cooled bythe primary heat exchanger, the secondary heat exchanger providing for asecond gas velocity substantially greater than the first; a waterstorage assembly including a tank having a baffle to separate a volumeof hot water from a smaller volume of cooler water at the bottom of thetank in communication with the hot water and a diffuser positioned overa cold water inlet into the bottom of the tank; means for circulatingthe cooler water through the secondary heat exchanger in counterflowheat exchange relationship with the combustion gases to cool the gasesbelow their dew point temperature and condense water vapor from thegases; and means for directing the water from the secondary heatexchanger through the primary heat exchanger to cool the combustiongases to a temperature which is above the dew point temperature.
 3. Awater heating assembly comprising:a heat exchanger and combustionchamber housing; a water-walled combustion chamber within the housingsurrounded by a primary heat exchanger, the products of combustion gasflow path past the primary heat exchanger providing a first gas velocitythrough the primary heat exchanger; a secondary heat exchanger withinthe housing for receiving gases cooled by the primary heat exchangerproviding for a second combustion gas velocity substantially greaterthan the first;a cool water inlet; means for circulating cool water fromthe cool water inlet through the secondary heat exchanger at a firstflow rate in heat exchanger relationship with the combustion gases tocool the gases below their dew point temperature and condense watervapor from gases; and a water flow path from the output of the primaryheat exchanger to its input for recirculating water in the primary heatexchanger, said water flow path and the primary heat exchanger forming aprimary heat exchanger loop; means for mixing water from the secondaryheat exchanger with a portion of the water from the output of theprimary heat exchanger and a pump in said primary heat exchanger loopfor pumping the water mixture through the primary heat exchanger at aflow rate greater than said first flow rate to cool the combustion gasesto a temperature which is above their dew point temperature.
 4. A waterheating system comprising:a water-walled combustion chamber surroundedby a primary heat exchanger, the product of combustion gas flow pathpast the primary heat exchanger providing for a first gas velocitythrough the primary heat exchanger; a secondary heat exchanger forreceiving gases cooled by the primary heat exchanger, the secondary heatexchanger providing for a second gas velocity substantially greater thanthe first; a water storage assembly including a volume of hot water anda smaller volume of cooler water in communication with the hot water;means for circulating the cooler water from said smaller volume ofcooler water through the secondary heat exchanger in heat exchangerelationship with the combustion gases to cool the gases below their dewpoint temperature and condense water vapor from the gases; means fordirecting the water from the secondary heat exchanger through theprimary heat exchanger to cool the combustion gases to a temperaturewhich is above the dew point temperature; and means for directing heatedwater from the primary heat exchanger to said volume of hot water insuch a manner as to avoid mixing with the smaller volume of coolerwater.
 5. In a liquid heater comprising an insulated storage vessel anda fuel-fired heat exchanger assembly in which liquid is circulated fromthe storage vessel to the heat exchanger assembly to be heated, and thenreturned to the storage vessel, the liquid heater characterized inthat:the heat exchanger assembly comprises a primary heat exchangerconsisting of cylindrical finned tubing coils surrounding a centralburner, the flow of combustion products from said burner beingessentially radially outward through the surrounding coil at a firstvelocity, the partially cooled combustion products being directed toflow in an axial direction at a second velocity substantially greaterthan the first over a secondary heat exchanger coil in which said gasesare cooled below their dew point temperature so as to extract some ofthe latent heat in said flue gases; liquid is withdrawn from the bottomof said storage vessel to be circulated first through said secondaryheat exchanger at a first flow rate in heat exchange relationship withproducts of combustion, then mixed with a portion of the liquid issuingfrom the outlet of said primary heat exchanger, and then caused, byaction of a circulating pump placed in a primary heat exchanger loop, toflow through said primary heat exchanger at a flow rate greater thansaid first flow rate; and the thus heated liquid is returned to an uppersection of the storage vessel in such a manner as to avoid mixing withthe liquid stored in the bottom of the storage vessel.
 6. A waterheating assembly as claimed in claim 3 wherein:the primary heatexchanger is a coil of tubing surrounding the combustion chamber andcombustion gases flow radially over that tubing; and the secondary heatexchanger is a coil of tubing and gases from the primary heat exchangerflow axially over the secondary coil.
 7. A water heating assembly asclaimed in claim 6 wherein the primary and secondary coils are coaxial.8. A water heating assembly as claimed in claim 7 further comprising atertiary heat exchanger located concentrically within the secondary heatexchanger wherein an unburned fuel/air mixture is pre-heated by thecombustion gases from the secondary heat exchanger.
 9. A water heatingassembly as claimed in claim 3 or 4 further comprising means forpre-mixing fuel gas and an amount of air sufficient for completecombustion of the gas.
 10. A water heating assembly as claimed in claim9 wherein the flow of the gas/air mixture into the burner is induced bya blower positioned downstream from the secondary heat exchanger.
 11. Awater heating system as claimed in claim 4 wherein:the primary heatexchanger is a coil of tubing surrounding the combustion chamber andcombustion gases flow radially over that tubing; and the secondary heatexchanger is a coil of tubing and gases from the primary heat exchangerflow axially over the secondary coil.
 12. A water heating system asclaimed in claim 1 wherein the primary and secondary coils are coaxial.13. A water heating system as claimed in claim 12 further comprising atertiary heat exchanger located concentrically within the secondary heatexchanger wherein an unburned fuel/air mixture is pre-heated by thecombustion gases from the secondary heat exchanger.
 14. A water heatingsystem as claimed in claim 4 or 13 further comprising means forpre-mixing fuel gas and an amount of air sufficient for completecombustion of the gas.
 15. A water heating system as claimed in claim 14wherein the flow of the gas/air mixture into the burner is induced by ablower positioned downstream from the secondary heat exchanger.
 16. Awater heating system as claimed in claim 4 wherein the water storageassembly is a tank separated into hotter water and cooler water volumesby a baffle.
 17. A water heating system as claimed in claim 16 furthercomprising a diffuser positioned over a cold water inlet into the bottomof the tank.
 18. A water heating system as claimed in claim 4 furthercomprising a water flow path from the output of the primary heatexchanger to its input for recirculating water in the primary heatexchanger, said water flow path and the primary heat exchanger forming aprimary heat exchanger loop, and means, including a pump in said primaryheat exchanger loop, for recirculating hot water through the primaryheat exchanger mixed with water from the secondary heat exchanger toraise the temperature of mixed water to a temperature above the dewpoint temperature of the combustion gases.
 19. A liquid heater accordingto claim 5 in which partially cooled products of combustion are directedfrom said secondary heat exchanger to flow axially through a tertiaryheat exchanger located concentrically within said secondary heatexchanger, product of combustion in said tertiary heat exchanger beingin heat transfer communication with the unburned fuel/air mixtureflowing towards said fuel burner.